Thermal transfer printing, in the context of the present specification, is used to mean a process in which an image on a carrier sheet is printed onto an image-receiving surface of an article by placing the image in contact with said surface of the article, and heating the image to transfer the image onto the article surface. Usually force is applied to exert pressure on the sheet and maintain the sheet in contact with the surface during printing.
The image is typically formed on the carrier sheet by a first printing step. In this case, the overall two stage process can be considered as a retransfer printing process. Retransfer printing techniques are commonly used for printing on articles other than flexible sheet material.
One example of a retransfer printing process is dye diffusion thermal retransfer printing. In a first stage, an image is formed by dye diffusion thermal transfer printing on a retransfer intermediate carrier sheet. Dye diffusion thermal transfer printing is a process in which one or more thermally transferable dyes are caused to transfer from selected areas of a dye-donor sheet to a receiver by thermal stimuli, thereby to form an image. This is generally carried out in a printer having a thermal head or laser energy source, depending on the kind of dye-donor sheet used. Using a dye-donor sheet comprising a thin substrate supporting a dyecoat containing one or more uniformly spread dyes, printing is effected by heating selected discrete areas of the dye-donor sheet while the dyecoat is pressed against a dye-receptive surface of a receiver sheet, thereby causing dye to transfer to corresponding areas of the receiver. The shape of the image transferred is determined by the number and locations of the discrete areas which are subjected to heating. Full colour images can be produced by printing with different coloured dyecoats sequentially in like manner, and the different coloured dyecoats are usually provided as discrete uniform panels arranged in a repeated sequence along a ribbon-shaped dye-donor sheet or ink ribbon. In retransfer printing, the receiver sheet is in the form of a retransfer intermediate sheet which comprises a supporting substrate having a dye-receptive imageable layer on one side, usually with a backcoat on the other side to promote good transport through the initial printer. Suitable retransfer intermediate sheets are disclosed, e.g, in WO 98/02315 (EP 912349). The image-carrying intermediate sheet formed in the first stage of the process is separated from the dye-donor sheet, and in the second stage of the process, is pressed against the article, with its image-containing layer contacting a dye-receptive surface of the article. Heat is then applied to effect transfer of the image, usually over the whole area of the image simultaneously. This is commonly carried out in a press shaped to accommodate the article. See, for example, the mug press disclosed in U.S. Pat. No. 5,643,387.
Thermal retransfer techniques are of particular applicability to the customisation of three dimensional articles such as mugs and tiles made of materials of high thermal stability that can withstand the temperatures involved without distortion. The carrier sheet can be printed by a desktop printer, and the press can retransfer the image in less than 5 minutes. Thus the method has advantages over direct printing by silk-screen, offset or gravure processes because it can add personalisation at point-of-sale.
In general, prior art thermal retransfer techniques can only be used with articles made of materials able to withstand the elevated temperatures (and possibly also elevated pressures) involved without distorting or breaking, typically being restricted to articles of metal or refractory materials such as ceramics. This applies, e.g., to the mug press disclosed in U.S. Pat. No. 5,643,387, which employs high pressures to ensure compliance of the press with the surface of the mug to be printed, restricting the approach to refractory objects of certain forms (having only simple curves, in only one plane) which do not distort or break at the temperatures and pressures involved.
The specification of British Patent Application No 0100330.0 filed 6th Jan. 2001 (pursued as PCT/GB02/00037) discloses apparatus for printing an image onto an image-receiving surface of an article by a thermal transfer printing process, the apparatus comprising a heat-conductive member comprising foamed polymeric elastomer material and having a surface shaped to be substantially complementary to the image-receiving surface of the article; and means for heating the member.
In use, a carrier sheet of material bearing an image to be printed is held in contact with the image-receiving surface of the article by the member, via the complementary surface of the member. Typically, a sheet is located on the article in an appropriate position and the member is brought into contact with the exposed face of the sheet and force applied to exert pressure on the sheet sufficient to promote wetting contact between the sheet and the member. The member is heated by the heating means, possibly being pre-heated to a desired temperature prior to contact with the sheet and/or prior to contact of the sheet with the article. The sheet is held in contact with the article for a suitable time with the member at a suitable temperature to print the image on the surface of the article. Suitable printing conditions can be readily determined by experiment. The member is removed and the article allowed or caused to cool to ambient temperature. The sheet is usually removed from the article surface.
Because heat is applied via the member, rather than being applied via the article, the article is exposed to less heat. The apparatus therefore enables printing to be performed on articles of a wider range of materials than had been possible hitherto. In particular, it is possible to print on articles of thermoplastic materials such as many widely used polymers such as acetal resin, acrylic resins, polycarbonates etc, provided the article is suitably supported when heated so the shape of the article is retained.
The apparatus accordingly desirably includes a support for the article, shaped to support or receive the article while leaving exposed the image-receiving surface thereof. In embodiments intended for use in printing articles of thermoplastic material, the support should be shaped to be complementary to a part of the article so as fully to support the article with a snug fit. The support will be discussed further below.
During printing, the sheet should be held firmly in contact with the article to produce a print of good quality. The apparatus therefore preferably includes suitable urging means, eg for applying force between the member and support, e.g. in the form of a spring loading arrangement or a hydraulic system.
The member comprises foamed material, ie. material including a plurality of small openings, pockets or voids within the body of the material. The voids are typically filled with gas, commonly air. The voids should be small (preferably having a maximum dimension less than 1 mm) and are ideally substantially uniform in size and are preferably reasonably uniformly dispersed through the material, for uniformity of properties, such that any variation in conductivity is not such as to produce prints with a variation in optical density. The presence of the voids, coupled with the elastomeric nature of the member material, mean that the member is relatively soft and has a degree of elasticity and so can, if necessary, deform to an extent (at least when heated) to enable the surface thereof to conform to the image-receiving surface of an article, thus providing uniform contact between the member and article. Provided the member is sufficiently compliant to allow good wetting contact with the sheet and article under compression, variation between the shape of the member and article surfaces is tolerable. Thus, while said surface of the member should be shaped to be substantially complementary to the image-receiving surface of the article, exact complementarity is not required as a degree of variation can be accommodated by the deformability of the member. The degree of variation tolerable will depend on factors including the degree of deformability of the member, the pressure applied in use and the complexity of shape of the image-receiving surface of the article. References to “substantially complementary” in this context should be construed accordingly.
The void volume of the material should be selected to give desired properties, and should be sufficiently high to provide the necessary elasticity without being so high as to adversely affect thermal conductivity. Generally, the void volume is in the range of 5 to 35% by volume, typically being about 10% by volume.
Suitable foamed polymeric elastomer materials are well known to those skilled in the art, and are commercially available or can be produced by known techniques. A foamed material can be created with a controlled void level by several means, including but not limited to mechanical entrainment of gas, adding volatile liquids, dissolving gases in one or more components, using a chemical reaction which evolves gas, adding a material which decomposes to evolve a gas during curing. For example, air-filled voids can be created by vigorously mixing material with air to generate an aerated paste which sets, eg on addition of a curing agent, to produce a solid foamed material.
The foamed polymeric elastomer material preferably comprises a foamed silicone resin, but other foamed polymers that exhibit sufficient flexibility under printing conditions may alternatively be employed. Suitable materials are commercially available.
The member comprising foamed polymeric elastomer material is heat conductive so that in use heat can be conducted through the member to an image-bearing sheet in contact with the article. It is desirable for the member to have relatively high thermal conductivity properties so that in use the sheet reaches the transfer temperature rapidly, thus avoiding the need for prolonged exposure of the article to elevated temperatures and so reducing the likelihood of thermal degradation of the article. The member preferably has a thermal conductivity of at least 0.2 W/m/K, and in general desirably has as high a thermal conductivity as possible provided other properties are not compromised.
The presence of voids in the polymeric elastomer material reduces the thermal conductivity of the material. In order to raise the thermal conductivity, preferably to a value greater than that of the polymeric elastomer material in unvoided, unfoamed condition (eg unfoamed silicone resin), it is convenient to include in the material particles of high thermal conductivity material, having a thermal conductivity greater than that of the polymeric elastomer material and typically having a thermal conductivity of at least 1 W/m/K. Suitable high thermal conductivity materials include metals and metal oxides. The particles should be reasonably uniformly dispersed through the material, for uniformity of properties. The particles are preferably spherical or near spherical in form and relatively uniform in size in order not to affect adversely the flexibility and deformability of the member. For the same reason, the particles are preferably relatively small, eg having a maximum particle size in the range 3 to 30 microns. The particles also desirably have a maximum dimension less than 0.2 mm to avoid the risk of surface roughness of the member that might deform and emboss the article in use. The particles are desirably present in sufficient quantity to ensure that the image reaches and stays at the transfer temperature for printing for the required period of time, without being present at such high levels that flexibility of the material is compromised. Suitable particle loadings can be readily determined by experiment. The particles are preferably present at a level of at least about 10% of the volume, more preferably at least about 15% or more of the volume of the material.
The member is preferably made of material that is substantially isotropic, having voids, and particles if present, reasonably uniformly distributed therethrough.
The means for heating the member conveniently comprises a heatable plate or jacket in contact with the member other than said shaped surface thereof. The plate or jacket may be heatable in known manner, eg incorporating or being linked to electrically operated heating elements.
Alternatively or additionally, the member be rendered electrically conductive by the incorporation in the foamed material of particles of electrically conductive material, preferably carbon, so that the member may be heated by the passage of an electrical current therethrough.
The apparatus preferably includes a support, as noted above.
The support preferably has a lower thermal conductivity than that of the member, eg being of the same base material as the member but not in foamed condition and without added particles of high thermal conductivity material. For instance, the support conveniently comprises unfoamed silicone resin. This is because, in use, heat flow through an article tends towards a steady-state value during printing. Unless the thermal conductivity of the support is less than that of the foam, the support will suck heat away from the article, making it harder for the article to attain a suitable temperature for printing and necessitating the use of higher printing temperatures, which is generally undesirable. It is nevertheless possible to produce satisfactory prints using apparatus in which the support has higher thermal conductivity than that of the member. At least in embodiments intended for use in printing on articles of thermoplastic materials, the support should be comparatively hard, rigid and non-deformable.
The voids in the foamed material serve to make the material more compliant and deformable. Hence the voids are used in the heat-conductive member on the printing side in order to allow good conformation between the member and the combination of carrier sheet and article to be printed. The voids have the undesirable side effect of reducing the thermal conductivity of the material. This effect is more than compensated by adding thermally conductive filler material to the composition. In the case of the support, the material should not be too soft and deformable, or it will not be capable of providing the desired mechanical support. For this reason, it is preferred that there should be no voids present. It is also desired that the thermal conductivity should be lower than that of the other member. The absence of voids is a disadvantage from this point of view. However, the desired result can be achieved by not incorporating the thermally conductive filler in the support.
The support is preferably not heated during printing, to reduce exposure of the article to heat, and also for more rapid cooling of the article after printing.
The apparatus may include cooling means for cooling the article after printing and after removal of the member. For instance, means such as a fan may be provided for directing a flow of cold air over the printed surface of the article. Additionally or alternatively, the support may have an associated coolable member.
The support is typically vertically below the member in use of the apparatus, but this is not essential.
The apparatus is typically designed for use with a particular article, with the member, and support if present, being tailored to the exact shape of that particular article.
The specification of International Patent Application No. PCT/GB02/00037 also discloses a method of printing an image on an image-receiving surface of an article, comprising holding a carrier sheet bearing an image to be printed in contact with said image-receiving surface of the article by means of a heat-conductive member comprising foamed polymeric elastomer material and having a surface shaped to be substantially complementary to said image-receiving surface of the article; and heating the member to print the image on said surface.
The image on the carrier sheet may be produced by a variety of different printing techniques, including dye diffusion thermal transfer printing, using appropriate carrier sheet material.
For example, a dye diffusion thermal retransfer printing process using a retransfer intermediate sheet as discussed above may be adopted. A retransfer intermediate sheet typically comprises a supporting substrate having a dye-receptive coating, or receiver layer, on one surface thereof, e.g. as described in EP 409514, with the other surface optionally carrying a coating to improve the friction, release or static properties of the sheet during printing and preferably also carrying the heat-resistant backcoat, as described in EP 912349 (WO98/02315). The substrate may comprise paper, possibly polyolefin-coated paper, with the preferred substrate comprising a laminated material prepared by laminating an opaque, voided polypropylene film to at least one side of a cellulose paper base material, e.g. as disclosed in JP 06-84119-B. The voids in the polypropylene film layer improve the compliance of the sheet.
The substrate is preferably not too stiff or inelastic, at least at printing temperatures and preferably also at ambient temperature, at least for use in printing on a compound curved surface (concave or convex) of an article, such as a part spherical surface having curvature in two planes, to enable the sheet to conform to the surface without creasing. It may be desirable to pre-heat the sheet prior to holding the sheet in contact with the article so that the sheet is in softened condition and more readily able to conform to the shape of the surface. Such pre-heating can be achieved by radiant heat from the member in heated condition, being held in close proximity to the sheet located on the image-receiving surface of the article (but not held in contact therewith), prior to contact of the sheet by the member and application of force to press the sheet into engagement with the article.
The image-receiving surface of the article may optionally be coated with suitable material to enhance susceptibility of the surface to printing, in known manner. Examples of suitable coating materials are given in U.S. Pat. No. 5,643,387 at column 2 lines 23 to 52. U.S. Pat. No. 4,943,684 and EP 721848 also disclose coating materials suitable for use on ceramic articles.
The method and apparatus disclosed in PCT/GB02/00037 are thus applicable to printing on articles of a wide range of materials, including thermoplastic materials as well as metals and ceramic materials, and in a wide range of shapes, including printing on flat surfaces as well as on more complex, curved shapes (concave or convex) including compound curves. This approach can thus be used for printing on articles such as casings of mobile telephones made of polymers such as polycarbonate that soften and would otherwise distort under printing conditions of temperature and pressure. With such articles even slight distortion is unacceptable because the article must meet tight tolerances to fit correctly with other components.
Thermal transfer printing of an image onto an article of plastics material, as compared with an article of a high thermal stability material such as ceramic or metal, is a much more difficult, sensitive process, requiring better temperature control in the article to avoid unwanted distortions resulting from flow of the plastic. Conversely, it is necessary to use relatively high transfer temperatures in order to produce good quality high density colour images. It is known to use a two stage process for thermal transfer of an image onto an article of plastics material, with the first stage involving dye transfer by sublimation and the second stage involving dye fixation by use of rapid infra-red heating: see FR 2728505.
JP 3132862 discloses a mass retransfer system in which an image on a carrier sheet is mass transferred to an article of plastics material. The article bears an image-receiving layer of lower softening point than that of the plastics material. The image-receiving layer is designed to provide high adhesion with the material to be printed, and so is unsuitable for receiving an image by a dye thermal transfer process where efficient release of the carrier and image-receiving layer is required.